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Title: The role of strain in incommensurate plagioclase feldspars
Author: Atkinson, A. J.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1999
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Using a novel seven-circle X-ray diffractometer with an area sensitive detector, the three-dimensional shape and the intensity of a pair of satellite reflections from an incommensurate sample (An65) was followed as a function of temperature. A large displacive component was found, in contrast to the widely accepted hypothesis that the incommensurate structure is a consequence solely of competing Al/Si ordering schemes. The presence of diffuse satellites in precession photographs of a highly disordered volcanic sample supports this result. On a mesoscopic length scale, domains of perfect periodic antiphase boundaries are seen in the TEM, delimited by rings of defects. The domain size is strongly composition dependent, rising to several thousand Å at An71, and falling to zero at ~An30, possibly indicating the compositional limit of the e2 structure. Infrared spectroscopy was used to probe the microstructural variations in natural and heat-treated plagioclase series. In comparing the macroscopic strain (cosg from lattice refinements) with a measure of local strain, w, the two incommensurate phases show different behaviour, with the e1 phase extrapolating to anorthite, and e2 to disordered albite. On this length scale, the behaviour of the e1 samples is indistinguishable from the I⁻¹ phase. Order parameter values for I⁻¹ ordering are proposed from the e1 structure as a result of this work and extrapolate to zero between An40 and An50. The autocorrelation function is applied to the spectra and used to generate a local order parameter which is found to scale well with previous measurements of enthalpy of solution. Experiments were also performed on the similar incommensurate structure found in synthetic anorthites, in which high degrees of short range order were found even in the macroscopically disordered state. The two sets of data form single trends as functions of degree of order.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available